Curable thermoset coating compositions are widely used in the coatings art. They are often used as topcoats in the automotive and industrial coatings industry. Such topcoats may be basecoats, clearcoats, or mixtures thereof. Color-plus-clear composite coatings are particularly useful as topcoats where exceptional gloss, depth of color, distinctness of image, or special metallic effect is desired. The automotive industry has made extensive use of these coatings for automotive body panels.
Color-plus-clear composite coatings, however, require an extremely high degree of clarity in the clearcoat to achieve the desired visual effect. High-gloss coatings also require a low degree of visual aberrations at the surface of the coating in order to achieve the desired visual effect such as high distinctness of image (DOI). Finally, such composite coatings must also simultaneously provide a desirable balance of finished film properties such as chip performance, durability, hardness, flexibility, and resistance to environmental etch, scratching, marring, solvents, and/or acids.
Chip performance or gravel resistance is particularly important in automotive coatings, especially those intended for use on automotive components with leading edges, such as rocker panels and front bumpers. Weak or poor resistance to chipping can result in significant damage to the overall vehicle appearance and greatly reduced durability.
Prior art automotive coating systems have long sought individual coatings or coating systems that provide such desirable chip resistance. It would be particularly advantageous to obtain a single coating that could be used to provide improved chip resistance in a wide variety of coating systems employing various types of topcoats.
The prior art has also long sought a way to improve chip resistance that is independent of the chemistry or molecular structure of the resin or binder component of a particular coating.
Of course, improvements in chip resistance must not be obtained at the expense of other important properties such as appearance and VOC.
In order to obtain the extremely smooth finishes that are generally required in the coatings industry, coating compositions must exhibit good flow before curing. Good flow is observed when the coating composition is fluid enough at some point after it is applied to the substrate and before it cures to a hard film to take on a smooth appearance. Some coating compositions exhibit good flow immediately upon application and others exhibit good flow only after the application of elevated temperatures.
One way to impart fluid characteristics and good flow to a coating composition is to incorporate volatile organic solvents into the composition. These solvents provide the desired fluidity and flow during the coating process, but evaporate upon exposure to elevated curing temperatures, leaving only the coating components behind.
However, the use of such solvents increases the volatile organic content (VOC) of the coating composition. Because of the adverse impact that volatile organic solvents may have on the environment, many government regulations impose limitations on the amount of volatile solvent that can be used. Increasing the percentage nonvolatile (%NV) of a coating composition or decreasing the VOC provides a competitive advantage with respect to environmental concerns, air permitting requirements and cost.
There is thus a continuing desire to obtain thermoset coatings having an improved chip performance while still possessing the optimum balance of performance properties required by the automotive industry. This optimum balance of performance properties in the finished film must be obtained without sacrificing the rheological properties of the coating composition required for trouble-free application of the composition while still maintaining the optimum level of smoothness and appearance.
It would be particularly advantageous to obtain coatings useful as primers that provide in improvements in chip resistance, especially in multilayer coating systems.